1.\" $NetBSD: sigaltstack.2,v 1.3 1995/02/27 10:41:52 cgd Exp $ 2.\" 3.\" Copyright (c) 1983, 1991, 1992, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)sigaltstack.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt SIGALTSTACK 2 38.Os BSD 4.2 39.Sh NAME 40.Nm sigaltstack 41.Nd set and/or get signal stack context 42.Sh SYNOPSIS 43.Fd #include <signal.h> 44.Ft int 45.Fo sigaltstack 46.Fa "const stack_t *restrict ss" 47.Fa "stack_t *restrict oss" 48.Fc 49.Sh DESCRIPTION 50.Fn sigaltstack 51allows users to define an alternate stack on which signals 52are to be processed. 53If 54.Fa ss 55is non-zero, 56it specifies a pointer to and the size of a 57.Em "signal stack" 58on which to deliver signals, 59and tells the system if the process is currently executing 60on that stack. 61When a signal's action indicates its handler 62should execute on the signal stack (specified with a 63.Xr sigaction 2 64call), the system checks to see 65if the process is currently executing on that stack. 66If the process is not currently executing on the signal stack, 67the system arranges a switch to the signal stack for the 68duration of the signal handler's execution. 69.Pp 70If 71.Dv SS_DISABLE 72is set in 73.Fa ss_flags , 74.Fa ss_sp 75and 76.Fa ss_size 77are ignored and the signal stack will be disabled. 78Trying to disable an active stack will cause 79.Nm 80to return -1 with 81.Va errno 82set to 83.Dv EINVAL . 84A disabled stack will cause all signals to be 85taken on the regular user stack. 86If the stack is later re-enabled then all signals that were specified 87to be processed on an alternate stack will resume doing so. 88.Pp 89If 90.Fa oss 91is non-zero, the current signal stack state is returned. 92The 93.Fa ss_flags 94field will contain the value 95.Dv SA_ONSTACK 96if the process is currently on a signal stack and 97.Dv SS_DISABLE 98if the signal stack is currently disabled. 99.Sh NOTES 100The value 101.Dv SIGSTKSZ 102is defined to be the number of bytes/chars that would be used to cover 103the usual case when allocating an alternate stack area. 104The following code fragment is typically used to allocate an alternate stack. 105.Bd -literal -offset indent 106if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL) 107 /* error return */ 108sigstk.ss_size = SIGSTKSZ; 109sigstk.ss_flags = 0; 110if (sigaltstack(&sigstk,0) < 0) 111 perror("sigaltstack"); 112.Ed 113An alternative approach is provided for programs with signal handlers 114that require a specific amount of stack space other than the default size. 115The value 116.Dv MINSIGSTKSZ 117is defined to be the number of bytes/chars that is required by 118the operating system to implement the alternate stack feature. 119In computing an alternate stack size, 120programs should add 121.Dv MINSIGSTKSZ 122to their stack requirements to allow for the operating system overhead. 123.Pp 124Signal stacks are automatically adjusted for the direction of stack 125growth and alignment requirements. 126Signal stacks may or may not be protected by the hardware and 127are not ``grown'' automatically as is done for the normal stack. 128If the stack overflows and this space is not protected 129unpredictable results may occur. 130.Sh RETURN VALUES 131Upon successful completion, a value of 0 is returned. 132Otherwise, a value of -1 is returned and 133.Va errno 134is set to indicate the error. 135.Sh ERRORS 136.Fn sigaltstack 137will fail and the signal stack context will remain unchanged 138if one of the following occurs. 139.Bl -tag -width [ENOMEM] 140.\" =========== 141.It Bq Er EFAULT 142Either 143.Fa ss 144or 145.Fa oss 146points to memory that is not a valid part of the process 147address space. 148.\" =========== 149.It Bq Er EINVAL 150An attempt is made to disable an active stack. 151.\" =========== 152.It Bq Er EINVAL 153The 154.Fa ss 155argument is not a null pointer, and the ss_flags member 156pointed to by 157.Fa ss 158contains flags other than SS_DISABLE. 159.\" =========== 160.It Bq Er ENOMEM 161The size of the alternate stack area is less than or equal to 162.Dv MINSIGSTKSZ . 163.\" =========== 164.It Bq Er EPERM 165An attempt was made to modify an active stack. 166.El 167.Sh LEGACY SYNOPSIS 168.Fd #include <sys/types.h> 169.Fd #include <signal.h> 170.Pp 171The include file 172.In sys/types.h 173is necessary. 174.Pp 175.Bd -literal 176struct sigaltstack { 177 char *ss_sp; 178 int ss_size; 179 int ss_flags; 180}; 181.Ed 182.Pp 183.Ft int 184.br 185.Fo sigaltstack 186.Fa "const struct sigaltstack *ss" 187.Fa "struct sigaltstack *oss" 188.Fc ; 189.Pp 190The variable types have changed. 191Specifically, the 192.Vt sigaltstack 193struct is no longer used. 194.Sh COMPATIBILITY 195Use of the (obsolete) 196.Vt sigaltstack 197struct will cause compiler diagnostics. 198Use 199.Vt stack_t , 200defined in 201.In signal.h . 202.Sh SEE ALSO 203.Xr sigaction 2 , 204.Xr setjmp 3 , 205.Xr compat 5 206.Sh HISTORY 207The predecessor to 208.Nm sigaltstack , 209the 210.Fn sigstack 211system call, appeared in 212.Bx 4.2 . 213